Interview Question for Senior Software Engineer at Western Digital:

It's a trade off question. There are two basic ways to do it.
1. A combination of looping, bitwise & and shifting, counting the ones as you go
2. Create a table of all the possible values for a byte, use this table and each byte of the number to lookup the value and sum.

1 is better when space constraints are tight, 2 is better when time constraints are more important.